Image forming apparatuses typically include a development device to develop latent images formed on a photoreceptor with developer, and two-component developer consisting essentially of toner (toner particles) and magnetic carrier (carrier particles) is widely used in image forming apparatuses. Development devices typically include a development roller or a development sleeve serving as a developer bearer and a developer conveyance member to transport the developer inside a developer conveyance channel in the development device.
Developer in the developer conveyance channel is supplied to the development sleeve, and, in a development range where the development sleeve faces a photoreceptor serving as a latent image bearer, toner in the developer is supplied to a latent image formed on the photoreceptor. Thus, the concentration of toner in the developer decreases.
Accordingly, if developer that has passed through the development range is returned to the identical developer conveyance channel from which developer is supplied (i.e., a supply channel), the concentration of toner decrease downstream in the supply channel. In particular, in images having high printing ratio, differences in the concentration of toner in developer between an upstream area and a downstream area from the development range is greater. Accordingly, it is possible that image quality is affected by the drop in the concentration of toner on the downstream side of the supply channel.
In view of the foregoing, providing multiple developer conveyance channels are proposed so that supply of developer and collection of developer that has passed through the development range can be performed in different developer conveyance channels (i.e., a supply channel and a collecting channel). The multiple developer conveyance channels are typically disposed parallel to the development sleeve.
Although such configuration is effective to inhibit the drop in the toner concentration on the downstream side of the supply channel, the amount (i.e., surface level) of developer decreases downstream in the supply channel because developer supplied from the supply channel to the development sleeve is not collected in the supply channel. Consequently, the amount of developer supplied to the development sleeve becomes uneven in the axial direction of the development sleeve. The unevenness in the amount of supplied developer can result in unevenness in image density.
To overcome such difficulties, various approaches have been tried. For example, in JP-H05-333691-A, the velocity at which developer is transported (hereinafter “developer conveyance velocity”) in the supply channel is increased so that the amount of developer transported therein is greater than the amount of developer supplied to the development sleeve.
Additionally, in JP-2006-251440-A, the developer conveyance member in the supply channel is screw shaped (i.e., a supply screw), and the blade pitch of the supply screw is reduced downstream in the supply channel. As the blade pitch decreases (narrows), the distance by which developer is transported per revolution of the supply screw decreases. Accordingly, the level of developer is higher in the area where the blade pitch is shorter when the amount of developer is not changed. Accordingly, the developer conveyance velocity on the upstream side in the supply comportment is higher than that on the downstream side.
However, increasing the developer conveyance velocity on the upstream side in the supply channel can cause aggregation of developer or unevenness in the amount of developer supplied to the development roller, resulting in substandard images.